1. Field of the Invention
The present invention relates generally to oilwell casing string joint locators, and more particularly, to a joint locator and methods for positioning a well tool connected to a length of coiled or jointed tubing in a well.
2. Description of the Related Art
In the drilling and completion of oil and gas wells, a wellbore is drilled into a subsurface producing formation. Typically, a string of casing pipe is then cemented into the wellbore. An additional string of pipe, commonly known as production tubing, may be disposed within the casing string and is used to conduct production fluids out of the wellbore. The downhole string of casing pipe is comprised of a plurality of pipe sections which are threadedly joined together. The pipe joints, also referred to as collars, have increased mass as compared to the pipe sections. After the strings of pipe have been cemented into the well, logging tools are run to determine the location of the casing collars. The logging tools used include a pipe joint locator whereby the depths of each of the pipe joints through which the logging tools are passed is recorded. The logging tools generally also include a gamma ray logging device which records the depths and the levels of naturally occurring gamma rays that are emitted from various well formations. The casing collar and gamma ray logs are correlated with previous open hole logs which results in a very accurate record of the depths of the pipe joints across the subterranean zones of interest and is typically referred to as the joint and tally log.
It is often necessary to precisely locate one or more of the casing pipe joints in a well. This need arises, for example, when it is necessary to precisely locate a well tool such as a packer or a perforating gun within the wellbore. The well tool is lowered into the casing on a length of tubing. The term tubing refers to either coiled or jointed tubing. The depth of a particular casing pipe joint adjacent or near the desired location at which the tool is to be positioned can readily be found on the previously recorded joint and tally log for the well. Given this readily available pipe joint depth information, it would seem to be a straightforward task to simply lower the well tool connected to a length of tubing into the casing while measuring the length of tubing inserted in the casing. Measuring could be performed by means of a conventional surface tubing measuring device. The tool is lowered until the measuring device reading equals the depth of the desired well tool location as indicated on the joint and tally log. However, no matter how accurate the tubing surface measuring device is, the true depth measurement is flawed due to effects such as tubing stretch, elongation due to thermal expansion, sinusoidal and helical buckling of the tubing, and a variety of other unpredictable deformations in the length of the tubing from which the tool is suspended in the wellbore. In addition, coiled tubing tends to spiral when forced down a well or through a horizontal section of a well.
A variety of pipe string joint indicators have been developed including slick line indicators that can produce drag inside the pipe string and wire line indicators that send an electronic signal to the surface by way of electric cable and others. These devices, however, either cannot be utilized as a component in a coiled tubing system or have disadvantages when so used. Wireline indicators do not work well in highly deviated holes because they depend on the force of gravity to position the tool. In addition, the wire line and slick line indicators take up additional rig time when used with jointed tubing.
Thus, there is a need for an improved joint locator system and method of using the tool whereby the locations of casing joints can be accurately determined, and the information transmitted to the surface, as the coiled or jointed tubing is lowered into a well.